Alveolar macrophages play a vital role in the inflammatory response to inhaled pathogens and particulates and, by that, are a crucial element of host defense. Further, they participate in antigen presentation and secrete biologically-active substances that modulate the growth and differentiation of lung cells and interact with other phagocytes. Despite the importance of alveolar macrophages in lung physiology and pathophysiology, there is limited knowledge of the factors that modulate and regulate their crucial effector functions (i.e., scavenger activity and secretory properties). Over the past decade, many studies have shown that intracellular pH (pHi) affects and is affected by cell functions. It is not surprising, therefore, that patterns of pHi regulation are cell- specific and "function-related". We recently identified a novel mechanism for pHi regulation in alveolar macrophages, namely, a plasmalemmal vacuolar-type H+-ATPase (V-ATPase) whose activity is allosterically activated with reductions in pHi. V-ATPase activity persists at low extracellular pH (pH0), where other mechanisms for pHi regulation (e.g., Na+-H+ exchange) may be limited. We postulate that the activity of plasmalemmal V-ATPase preserves pHi and, therefore, the ability of alveolar macrophages to maintain pHi-sensitive functions in regions of acidic pH0 (e.g., tumors and abscesses). The proposed research seeks insight into the links between macrophage effector functions and the activity of plasmalemmal acid-base transporters. Our strategy is to correlate selected markers of macrophage functional competence with the activity of plasmalemmal acid-base transporters. Studies will be conducted to characterize the biophysical determinants of specific acid- base transporters, as a function of pHi and pH0. Tightly coupled to these measurements, quantitative theoretical models of whole cell PHi regulation will be used to characterize the kinetic properties of the constituent acid-base transporters. Hypothesis-driven studies then will be conducted to define the mechanisms underlying the responses of individual acid-base transporters to selected exogenous agonists, namely endotoxin and phorbol esters. Concurrently, the pHi/pH0-sensitivity of selected effector functions (phagocytosis, TNFalpha release, superoxide generation) will be determined using a variety of alternate methods to verify the specificity of the observed effects. These tightly-coupled measurements will clarify and quantify the relationships between pHi regulation and macrophage functional competence in resident alveolar macrophages. The results should provide important insights into the mechanisms that modulate alveolar macrophage functions in health and the derangements associated with pulmonary infections and acute lung injury.